Power bank is a portable device that can be both charged and discharged, which charges digital devices such as cell phones when needed. A power bank normally comprises a battery and a control circuit thereof. FIG. 1 schematically shows a block diagram of a prior art power bank 10. As shown in FIG. 1, the power bank 10 comprises: an input-output port 11, a battery port 12 and a charge-discharge circuit 13, wherein the input-output port 11 is coupled to a power source or a digital device, the battery port 12 is coupled to a battery 14. Typically, the battery 14 is a single battery (1-cell), which has a 3.7˜4.2V voltage level when fully charged; the power source (e.g., an adapter) provides an input voltage which has a 5V voltage level; and the digital device has a 5V voltage level supply requirement. So when the input-output port 11 is coupled to the power source, the charge-discharge circuit 13 operates at step-down (e.g., buck) mode, so that the battery 14 is charged by the power source; and when the input-output port 11 is coupled to the digital device, the charge-discharge circuit 13 operates at step-up (e.g., boost) mode, so that the battery 14 is discharged, to supply the digital device.
With the increase of the consumer's requirement, more power provided by the power bank is desired. One example is to replace the single battery 14 with two series coupled batteries (i.e., to replace 1-cell with 2-cell). However, when the input-output port 11 is coupled to the power source with 5V voltage level, if the charge-discharge circuit 13 still works at step-down mode, the two series coupled batteries can not be fully charged; and when the input-output port 11 is coupled to the digital device, if the charge-discharge circuit 13 still works at step-up mode, due to the voltage across the two series coupled batteries is higher than the voltage requirement of the digital device (i.e., the input voltage is higher than the output voltage), the step-up mode could not be started up, so the digital device could not get power supplied.